The proposed research deals with the neuronal control of neck movements by the vestibulospinal system. Such movements play an important role in postural reflexes and the orientation of cranial sense organs, but we do not know how these movements are organized within the cervical spinal cord. Abnormalities in vestibulocollic function in man are associated with deficits in postural regulation and are common sequelae of clinical conditions such as Meniere's disease. To understand vestibular control of neck movements we need to know: (1) the neuronal populations which make up the vestibulospinal system; and (2) the synaptic relations of each population with cervical motor neurons. In experiments already completed or underway I am describing cervical motor neurons. In experiments already completed or underway I am describing cervical motor pools and the ultrastructure of first order afferents to cervical motor neurons in Pseudemys scripta elegans. The proposed research will extend this analysis of cervical muscle control. (1) Vestibulospinal axons will be labelled with horseradish peroxidase (HRP) and characterized in terms of four structural parameters which are known to have functional significance: axon diameter, terminal arbor morphology, branching pattern, and synaptic bouton ultrastructure. (2) The synaptic relations between HRP labelled vestibulospinal axons and cervical motor neurons will be assessed under the electromicroscope. The object of these experiments is to identify the various morphological populations of neurons which relay vestibular signals to cervical motor neurons and to further our understanding of the functional role which each of these populations plays in the organization of neck movements. This research, combined with work in progress, will contribute to our understanding of cervical control in two ways. The proposed research will provide the first comprehensive and systematic morphological description of vestibulo-cervical axons in any vertebrate. Second, the proposed research will provide important data on the mechanisms underlying recruitment of cervical motor neurons.